I recently reformatted my computer and reinstalled Win 7 Ultimate x64 and was having problems with random programs that I was attempting to install from certain installation/exe packages, downloaded or otherwise (Winamp 2.91 as an example...I had to download the current version to install it which is 5.0+). Installation packages would simply cause the 'Stopped Working' troubleshooting dialog to popup, and the options available to troubleshoot the problem would simply close the dialog.
Simultaneously, I was unable to install Flash Player using the smaller downloader exe file from the Adobe site (http://get.adobe.com/flashplayer/otherversions/) as I would doubleclick the file, select 'Run' in the 'Security Warning' dialog, and the installation package would crash, giving the same error above. Instead, I installed Flash using an old, stand-alone installation package (version 10) that I had backed up and it installed fine, however, sites like Youtube or other live streaming sites would immediately crash, asking me to update Flash. I found various sites stating that the solution was to disable 'Hardware Acceleration' which would fix the issue and it did somewhat, but there were still errors with some Flash-enabled sites (NHL.com's live stream of a game would only allow me to watch at the lowest bandwidth setting).
After many installations, uninstallations, reinstallations, reboots, etc., I was able to figure out that my problem was caused by UAC (User Accounts Control) built within Win 7: I had disabled UAC immediately after I reinstalled Windows as I had always had it disabled for I had no use for it and disliked the incessant popup messages.
After re-enabling UAC and rebooting, I was able to reinstall Flash using the official Adobe downloader and now I am able to normally use Flash Player on enabled sites.
Whether or not this is a problem caused by either Adobe or Microsoft, I wouldn't know, but it has seems to be rectified with this solution. This is after believing that my Firefox addons, such as NoScript and AdblockPlus were causing the problems I was having (initially probably exacerbating the problems...and annoying the hell out of me...but clearly not the root of the problems).
Hopefully this helps some people.
Other System Specs and Comments (hardware has nothing to do with it):
-Win 7 x64 Ultimate, with latest updates.
-Flash Player 220.127.116.11 for x64/Firefox
-AdBlock Plus 2.0.3
-NHL.com now allows me to watch at the highest bandwidth setting
-Justin.tv no longer crashes
-Youtube no longer crashes
-Hardware Acceleration is enabled
-Winamp 2.91 installed just fine
I installed Flash using an old, stand-alone installation package (version 10)...
You can find the links to the full installer packages for Flash Player 11 at http://forums.adobe.com/thread/909550